Workers in the art of making and using disk drive recording equipment are aware of certain shortcomings in the associated servo techniques and apparatus therefor. At times in a disk drive servo workers contemplate the use of a "phase lock loop" [PLL]. It is evident that "loss of lock" in a PLL arrangement can create many undesirable problems. For instance, when the associated phase lock oscillator [PLO] "unlocks", it is usually desirable (if not necessary) to protect the integrity of reported data by disabling the recorder's write current, to prevent erasure of recorded data. The subject invention addresses this problem, providing means which can quickly and accurately determine loss of lock in a phase lock loop, thus making it possible to so disable the recording means. Also, the invention makes it possible to disable power to the head-disk assembly [HDA] to prevent any damage thereto.
The problem of detecting a "lock" for a phase lock loop has been addressed before--e.g., see Memorex Disk Drive Model 3680 wherein a servo channel was arranged to monitor phase error voltage between the voltage-controlled oscillator [VCO] and the carrier signal. If this voltage exceeded a preset reference voltage, PLL "unlock" was assumed. The present invention teaches a different, and somewhat superior, method of detecting "lock" and "unlock" of a phase lock loop in a disk drive servo system.
Thus it is an object of the invention to address the above mentioned problems and difficulties and to provide at least some of the afore-described features and advantages. A more particular object is to provide a different, somewhat superior, technique for detecting loss of phase/frequency lock in a phase lock loop. A related object is to do so in a phase lock oscillator [PLO] "unsafe detection" circuit. Yet a further object is to provide the above by monitoring timing between a voltage-controlled oscillator clock and the digitized carrier signal in a PLO arrangement. A further related object is to so monitor to determine if the VCO "locks" to a harmonic of the carrier signal, whereupon timing deviation can be immediately detected. A further object is to provide the foregoing in a circuit which also detects any frequency-lock whether phase-locked or not. Insert the standard list of figures.